The wind energy field is encountering steady growth aimed to meet a worldwide increasing demand for green energy. The research and development of the new technology focused on improving the quality, cost, efficiency, and reliability of the machines for producing such green energy is consequently very active worldwide.
One of the latest trends is a so called direct drive arrangement, wherein the rotor blade directly drives the generator with no gearbox, interposed between. This direct drive arrangement is introducing a major improvement in efficiency and reliability because gearboxes are inefficient and high maintenance items, and are basically negatively affecting original equipment costs as well operating costs (maintenance costs).
In the direct drive arrangement, however, one of the most challenging goals is to set the air gap distance between rotor and stator to minimal value in order to minimize the generator cost but also make sure that the deformations under wind loads do not create the condition of contact between rotor and stator which would unavoidably create a damage to the generator assembly. Such a condition can be defined in terms of the air gap “AG” between the rotor and the stator. When AG=0, the rotor has deflected to such a degree so as to contact the stator, and thus damage or entirely destroy the generator.
It is important to observe that direct drive technology can be classified according to Electric generator type as well as Mechanical drive train arrangement type. Generator type can be synchronous generator, asynchronous or a modern permanent magnet generator (PMG), while mechanical drive train arrangement can be classified in
Type 1 generator assembly directly coupled to the main shaft of the wind turbine,
Type 2 generator assembly coupled to main shaft assembly by means of suitable torque couplers
Type 1 arrangement foresees the generator assembly rigidly mounted to the wind turbine frame, the stator is stationary, the rotor is directly coupled to turbine main shaft end and consequently it is deforming due shaft deformation under wind loads. In this arrangement air gap between air gap generator stator motor must take into consideration the air gap contraction due to main shaft deformations induced by the wind loads.
Type 2 arrangement foresees the generator assembly rigidly mounted to the wind turbine frame. The generator has its own shaft and its own bearings and the generator shaft does not see the turbine main shaft deformations under wind load because a suitable coupling isolates the two shafts and pure torque is transmitted to the generator shaft (not undesired side loads). This type is rather rare due to the heavy cost and weight of the coupling.
As mentioned above, among the different type of electric direct drive generators, one of the most of the modern is the so called PMG (permanent magnet generator) wherein the efficiency, the size, the weight and ultimately the cost of the generator is greatly affected by the air gap value. For example, a minimum air gap of 3.0 mm on a 3 m diameter, 2 MW generator would be optimal to minimize generator electric active component mass and any additional millimeter (4.0 mm, 5.0 mm, etc.) of air gap would significantly increase dimension, weight, and cost.
On the other hand, the final generator air gap design value has to take into consideration a plurality of factors that are in reality demanding to increase said air gap design values, in particular:
Manufacturing tolerance
Shaft, bearings, and structure deformations under severe wind dynamic loads
Magnetic forces exchanged between rotor and stator
Additional temperature deformations induced from uneven temperature distribution
Since the accidental contact between rotating rotor (carrying magnets) and stationary stator would lead to almost immediate generator damage, it becomes evident how important it is that the structure of the aforementioned direct drive generators will be able to contain all above mentioned deformations within a minimum value in order to minimize the air gap requirements, thus allowing the generator to be efficient from the electric standpoint.
Of course building rigid structures means heavy weights, high rigidity bearings, and ultimately, higher costs. It is consequently evident that the air gap value must be set to a compromised value which minimizes the overall cost of the generator electric active components and mechanical structure all together (increasing air gap reduces structural costs but increases generator costs and vice versa).
All above factors contributing to the need to increase said generator air gap are in direct contrast to the need of minimizing it in order to achieve minimum generator cost and maximum efficiency. An example will help to explain
Mechanical manufacturing tolerance AG1=2.0 mm
Wind deflection AG2=2.0 mm
Electro-magnetic forces AG3=1.0 mm
Temperature induced deformations AG4=1.5 mm
Total air gap requirement=AG1+AG2+AG3=6.5 mm
Optimal air gap distance to achieve the lowest cost/best performance electric generator=AG0=2.5 mm
Additional costs of the generator to provide the same power operating with AG=6.5 mm versus AG=2.5 mm is estimated to be at least 20%, with the additional disadvantage of the efficiency dropping at least two points.
Consequently it appears clear, how an air gap higher than optimum value affects the costs of the electric active masses inside the generator of the wind turbine. On the other hand, tightening the air gap down to generator optimal value (2.5 mm) while maintaining safe operations for 25 years life under any load condition without ever reaching the dangerous condition AG=0 (contact between rotor and stator, and generator damage) would mean adapting very stiff, heavy and expensive mechanical structures, which would be far more expensive than the extra generator cost and efficiency drop mentioned above.
This is basically the reason why most of the known PPM direct drive generators for wind turbines (almost entirely coupled to turbine main shaft according to type 1 arrangement mentioned above) are operating within ‘compromised’ air gap values between 5 and 8 mm, which is rather distant from optimum electric generator cost and performances.
Recently, the higher cost of the generator due to increased air gap operating conditions is further aggravated from the higher cost of magnets (rare earth magnets, such as neodymium) which recently have procurement costs far higher than in the past. A generator operating with higher than optimal air gap would need higher neodymium magnets in high quantity to produce the same power (to compensate drop of efficiency due to increased air gap).
Accordingly, there is a need in the art for a wind turbine having a generator that provides a high output solution at a relatively low cost by virtue of a reduced air gap. Embodiments of the invention provide such a generator.
The invention provides such a wind turbine and associated generator. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.